VST

VST plugins generally run within a DAW to provide additional functionality. Most VST plugins are either instruments (VSTi) or effects, although other categories exist—for example meters or analyzers. VST addons generally provide their own GUI (graphical user interface) to display knobs, keyboard or other features onscreen.

VSTi can simulate original equipment, so a software copy of a keyboard/synthesizer or sampler. This way you may can purchase the instrument you always wanted to have but had no space to store it or not enough cash to buy it.

Generally the usage of a VST makes your life easier and speeds up lenghty development and patching process within your DAW.